Update index creation message
Update index creation message

This change rewrites the message at the end of the script to be more
consistent with the README file. The README says to run the
script but not to modify the config file until after running
. The ending message states these instructions,
uses the replace-with key, and provides a URL to the README
for convenience.

rust-lang/crates.io,steveklabnik/crates.io,Susurrus/crates.io,rust-lang/crates.io,Susurrus/crates.io,achanda/crates.io,steveklabnik/crates.io,Susurrus/crates.io,rust-lang/crates.io,achanda/crates.io,rust-lang/crates.io,Susurrus/crates.io,steveklabnik/crates.io,achanda/crates.io,steveklabnik/crates.io,achanda/crates.io